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Nervous System CNS: coordinating center for Mechanical and chemical reactions Made up of brain and spinal cord PNS: all parts of nervous system EXCEPT for brain and spinal cord relays information btw CNS and other parts of body Handout LSM 9.1-1 Types of Nerve Cells • Types of nerve cells – Glial cells – non-conducting, provide structure – Neurons – functional units – Sensory neurons - carry impulses from sensory receptors to CNS, (aka afferent neurons) – Motor neurons –carry impulses from CNS to effectors (muscles, organs and glands) (aka efferent neurons) – Interneurons – link neurons in the body Anatomy of a Nerve Cell Handout LSM 9.1-3 nodes of Ranvier –gaps between myelin sheath along the axon Dendrites carry impulses Towards cell body Myelin sheath – insulating covering over axon, prevents loss of charge, composed of Schwann cells Axon - Carries nerve Impulses away from cell body Neural Circuits Example - Reflex Arc Stimulus – touching a hot stove or touching a tack Response – arm pulls away from the stimulus quickly Handout LSM 9.1-5 Electrochemical Impulse SBI 4U Past Research • Luigi Galvani (late 18th c.) – leg muscle of dead frog twitched from electrical stimulation • Emil DuBois-Reymond (1840) – refined instruments to detect currents in nerves and muscles • Julius Bernstein (1900) – suggested nerve impulses are electrochemical messages created by ion movement through nerve cells • Willem Einthoven (1906) – recorded transmission of electrical impulses in heart muscle (ECG) • Hans Berger (1929) – measured electrical activity of brain with electrodes (EEG) K.S. Cole & H.J. Curtis (1939) • resting potential = voltage difference across a nerve cell membrane during the resting stage (-70 mV). • action potential = the voltage difference across a nerve cell membrane when the nerve is excited (+40 mV). Resting Potential • resting membrane is about 50x more permeable to K+ • more K+ diffuse out of nerve cell than Na+ diffuse in • exterior becomes +ve relative to interior • https://www.youtube.com/watch?v=9euDb4 TN3b0 Terminology • polarized membrane = membrane charged by unequal distribution of positively charged ions inside and outside the nerve cell. • depolarization = diffusion of sodium ions into the nerve cell resulting in a charge reversal. • sodium-potassium pump = an active transport mechanism that moves sodium ions out of and potassium ions into a cell against their concentration gradients. • repolarization = process of restoring the original polarity of the nerve membrane. The Action Potential a) K+ diffuses out faster than Na+ into nerve cell. Outside = +ve, inside = -ve b) Electrical disturbance opens Na+ gates and Na+ rush into nerve cell. Membrane becomes depolarized. c) Depolarization closes Na+ gates, while K+ gates open. K+ follows concentration gradient and moves out of nerve cell. Action potential moves forward. d) Na+-K+ pump restores polarization (3 Na+ out to 2 K+ in). refractory period = recovery time required before a neuron can produce another action potential. Movement of the Action Potential Movement of a Nerve Impulse Threshold Levels and the All-or-None Response • • stimuli < 2 mV do not produce muscle contraction increasing intensity will not produce an increased response Threshold Levels and the All-or-None Response • • • • threshold level = minimum level of a stimulus required to produce a response. threshold levels are different for each neuron increasing the intensity of the stimuli above the critical threshold value will not produce an increased response – known as an all-ornone response. Intensity of stimuli is detected in 2 ways: – difference in frequency – the number of neurons that fire (different threshold levels) https://www.youtube.com/watch?v=SdUUP2pMmQ4 The Synapse Synaptic Transmission • synapse = regions between neurons, or between neurons and effectors (a.k.a. synaptic cleft). • neurotransmitters = chemicals released from vesicles into synapses. • presynaptic neuron – neuron that carries impulses to the synapse. • postsynaptic neuron – neuron that carries impulses away from the synapse. Synaptic Transmission 1. 2. 3. 4. 5. action potential moves along axon of the presynaptic neuron toward the synapse vesicles release neurotransmitter from end plate into synaptic cleft neurotransmitters diffuse across cleft and bind with receptors on postsynaptic neuron enzymes remove neurotransmitters from receptors and are either reabsorbed or broken down dendrites of postsynaptic neuron are depolarized and action potential continues Acetylcholine • a.k.a. “ACh” • a neurotransmitter found in the end plates of many neurons • can act as excitatory by opening Na+ channels • broken down by cholinesterase so that the neuron can recover – insecticides block action of cholinesterase so that the insects heart contracts but never relaxes An Excitatory Synapse Acetylcholine • can also act as inhibitory: – ACh opens K+ gates on some postsynaptic neurons – rush of K+ outside cell increases number of positive ions outside relative to inside – membrane becomes hyperpolarized, i.e. inside of the nerve cell membrane becomes more negative than the resting potential Summation • = effect produced by the accumulation of neurotransmitters from two or more neurons Other Neurotransmitters: • • • • • serotonin dopamine gamma-aminobutyric acid (GABA) glutamic acid norepinephrine … interactions of excitatory and inhibitory neurotransmitters is what allows us to react to our environment! Neurotransmitter Disorders • Parkinson’s disease – inadequate production of dopamine resulting in involuntary muscle contractions and tremours • Alzheimer’s disease – decreased production of ACh has been associated with the deterioration of memory and mental capacity Homework • Section 9.1 – Copy table 1 on page 416 – Page 416 Question # 1, 2 4-6 • Section 9.2 – Page 426 Question # 2-13